An exemplary embodiment of this application relates to an electrostatic powder coating process and apparatus for coating an article with charged powder particles by forming a powder cloud with one or more air jets, so that the cloud of charged particles surrounds and coats the article. More particularly, the exemplary embodiment relates to an electrostatic powder coating process and apparatus for coating a grounded article with triboelectrically charged powder particles contained in a chamber, advancing the charged powder particles onto at least one rotatable donor roll, rotating the donor roll past an air jet whereat a combination of electric fields and/or air jets detach the charged powder particles from the donor roll to form a powder cloud that is directed to the grounded article.
The process of coating metal or conductive surfaces with dry powder coatings is well known. The process has been used since the mid 1950's. The initial applications of electrostatic powder coating involved the coating of pipe and electric motors. By utilizing the natural principle of opposite charges attract, this coating technology offered manufacturers an alternative to solvent-based paints. With the growing need to reduce air pollution from solvent-based paints, the demand for electrostatic powder coating has increased over the years. Although the cost of powder and liquid coatings is comparable, dry powder coating is advantaged because spray booths are easily cleaned, no solvents are used eliminating the need for air pollution control equipment, over spray can be collected for reuse, thicker film coatings can be obtained in a single application, and powder has no surface tension, so it will penetrate into small gaps precluded by liquid coatings.
Powder particles for powder coating apparatus are typically delivered to spray guns that electrically charge the expelled powder particles by means of ion corona discharge or triboelectric charging. The powder feed system for the spray guns generally requires fluidization of the powder particles. The powder generally does not contain surface flow additives to improve fluidization because the quality of powder coating film would be compromised during the oven-curing step. With this materials constraint, the powder coating industry generally uses powder particles having an average diameter of 30 to 40 μm, since particles of this size are easily fluidized. The fluidized powder particles are pneumatically transported to a triboelectric or ion charging spray gun for charging the powder and directing the cloud of powder particles to the article to be coated. For triboelectric spray guns, the powder collides with triboelectric-active materials placed in a tortuous path within the gun. For ion spray guns, a voltage of approximately 100 kV applied to a needle electrode generates a corona and the ions are captured on the powder particles in the powder cloud. However, to achieve higher coating quality with thinner layers, there is a need for an improved process and apparatus for powder coating articles with powder particles having an average diameter of approximately 10 μm. This requirement is particularly desired for top gloss coat in automobile coating.
There are many existing powder coating systems wherein a powder is air fluidized in a reservoir and pneumatically fed to a spray gun where the powder is charged. A combination of electrostatic and pneumatic forces transports the charged powder to the article to be coated. Electrostatic forces attract the charged powder to the article. The coated article is typically baked in an oven for approximately 10 minutes at about 400° F. The powder coating melts and flows into a durable film. Such typical powder coating systems are provided by, for example, Wagner International AG, Ransburg Corporation, Nordson Corporation, and Gema Volstatic AG.
U.S. Pat. No. 6,342,273 discloses an improvement over the typical powder coating processes and apparatus. In this reference, electrophotographic development system technology is described to triboelectric or induction charge powder paint particles for electric field transfer directly to flat substrates for subsequently curing, such as, a continuous or discontinuous band, sheet or web substrates. One substrate example was an unwound flat coil. According to a preferred embodiment, the powder paint particles are mixed with magnetic or non-magnetic carrier particles to obtain friction charging. Then the mixture is transported adjacent the substrate to be coated by a transport means, and the charged powder particles are extracted from the mixture and applied to the substrate by means of an electric field between the substrate and the means of transport. The advantage of such a process is that it is possible to apply powder particles having particle sizes between 5–30 μm. Unfortunately, such a process is limited to coating directly onto closely adjacent and confronting flat substrates in a manner similar to developing an electrostatic latent image on a photoreceptor in the field of electrophotography. Thus, the process of this patent cannot adequately coat three-dimensional articles, such as those articles needed in the industry for appliances, automobiles, and the like.
U.S. Pat. No. 5,518,546 discloses process and apparatus for applying electrostatically charged powder resin particles to a substrate to be coated by utilizing a fluidized bed for inductively charging the powder resin particles through a high voltage means disposed at one portion of the fluidized bed and a grounded electrode disposed in another portion of the fluidized bed, so that an electric field is created therebetween. Fluidizing air is applied to the powder resin particles to establish an electrostatically charged powder cloud within the fluidized bed, and then pneumatically conveying the charged powder particles from the fluidizing bed to a dispensing nozzle or gun for directing the charged powder particles onto the substrate. The coating on the substrate is subsequently fused or cured to form a permanent film thereon. The powder resin particles are mixed with at least one modifying agent that promotes charging of the mixed particles, but does not alter the melt or durability characteristics of the powder resin particles.
U.S. Pat. No. 5,078,084 discloses using a powder material for coating large objects, such as, vehicle bodies. An electrical charge is applied to sprayed powder which is directed toward grounded vehicle bodies to be coated by spray guns. The application of powder material onto the automotive or truck bodies is performed in a spray booth, so that over spray powder that is not deposited on the vehicle body can be collected. An exhaust system that creates a negative pressure in the spray booth aids in containment of the over spray powder and causes the over spray powder to be drawn into a powder collection and recovery system. The recovered powder is saved for reuse by the powder spray guns.
U.S. Pat. No. 5,743,958 discloses apparatus to collect and reuse over spray powder that contains a proportionately greater percentage of smaller particles or “fines” than virgin powder, since a greater percentage of larger particles have adhered to the object to be coated. The system for applying powder coating material onto large objects, such as, vehicle bodies, includes a powder spray booth, a powder kitchen containing a mixing hopper, and a number of feed hoppers, which receive the powder material from the powder kitchen and supply it to powder spray guns. The mixing hopper in the kitchen maintains the selected ratio of recovered over spray powder and virgin powder material.
U.S. Pat. No. 4,805,069 discloses an electrostatic powder painting apparatus including a powder charging apparatus therein. The powder painting apparatus has a pair of plasma electrodes disposed in an insulative tubular passage for transporting powder carried by gas. A DC voltage is intermittently applied between the plasma electrodes to form one space where mainly desired polarity ions exist that are drawn from the plasma electrodes and another space where mainly opposite polarity ions exist, thereby assuring stable and strong charging without adhesion and accumulation of powder to and on either one of the pair of plasma electrodes.
U.S. Pat. No. 4,330,567 discloses an electrostatic fluidizing bed coating apparatus for work pieces, especially those of continuous length, such as metal wires. The coating apparatus includes a housing with a planar horizontal porous support member therein defining a fluidization chamber in the housing above the porous support member and a plenum below it. Gas is introduced into the plenum for passage upwardly through the porous member to effect fluidization of particulate coating material in the chamber. A means is provided for ionizing the gas in the plenum to effect electrostatic charging of the fluidized coating material. An electrically conductive grid is mounted in the plenum between the porous support member and ionizing means and has means to control its electrical potential. The work piece is passed through the housing between the porous support member and the cloud control grid, so that the grid may be used to affect the deposition of powder upon the work piece.
U.S. Pat. No. 3,680,779 discloses an electrostatic powder sprayer for depositing powder material on an article to be coated. A metering roller at the bottom of a powder reservoir is rotated to dispense powder. A first circuit is connected to the metering roller and a conductor located between the article and the metering roller establishes a primary AC electric field to accelerate the powder from the metering roller. At least one electrode is spaced a greater distance from the metering roller than the conductor. A second circuit is connected between the metering roller and the electrode to establish an auxiliary electric field therebetween. The second circuit is provided to further control the movement of powder from the area of the primary electric field to the article, so that a more uniform powder is laid on the article and the amount of powder “fly around” is reduced.
U.S. Patent Application Publication No. 2002/0127332 discloses apparatus and method for applying powder to the interior surface of a hollow object. A powder discharge device having a powder discharge outlet is positioned within the hollow object and a stream of electrostatically charged powder is directed through the discharge outlet. The object is rotated, so that the stream of charged powder contacts the interior surface of the rotating object.
U.S. Patent Application Publication No. 2002/0160123 discloses a method for electrostatic powder coating or painting of non-conductive polymer surfaces by applying a conductive layer, such as a metal foil on the side opposite the non-conductive polymer surface to be painted, so as to provide sufficient conductivity to enable electrostatic powder painting thereof. With the conductive layer in place, the non-conductive article can be painted electrostatically. After painting, the conductive layer can be optionally removed without affecting the painted surface.
U.S. Patent Application Publication No. 2003/0003813 discloses metallic studs of a dynamoelectric machine that are coated with a powder resin to form an electrical insulator between the rotor and the studs. The powder coating is applied by several disclosed methods. In the preferred method, an electrostatic fluidized bed is provided and air is passed through the powder disposed in the bed to obtain a rolling boil of the powder. An applied high potential is provided in the air to produce free electrons that are passed through the powder. The stud is located in the powder cloud in the fluidized bed and as the powder is charged, the electrostatic potential enables the powder to be attracted to the studs. The powder coating on the stud is then cured. Instead of disposing the stud in a powder cloud, a spray gun may be utilized.